Mutations in oncogenes and tumor suppressor genes in experimental neoplasms in animals and in naturally occurring cancers and precursor lesions in humans are defined and characterized to define the pathogenesis of specific tumors and to clarify the roles of carcinogenic agents in the progression of neoplasms to malignancy. K-ras and p53 gene mutations have been characterized both in differentiated primary gastric adenocarcinomas in a high-risk population in the vicinity of Florence, Italy, and in endometrial carcinoma of the uterine corpus and its precursor lesions in Japanese patients. The existence of multiple p53 pseudogenes in the rat has been demonstrated, and their sequences and chromosomal locations partially characterized.